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I am suggesting that commenting on blogs purely for the purpose of getting back links is spamming.

Purely for that is spamming - for sure. It's also easy to see who's doing that and simply not approve the comments.

It's not just on dofollow blogs though. As mentioned elsewhere on Sitepoint, nofollow links also have some SEO value. So, anyone commenting purely for backlinks is spamming - nothing to do with looking for dofollow or nofollow per se.

I have a problem with the nofollow tag. In a nutshell, it's a tag that users can't see and thus are a form of cloaking. I would think a site owner would like to protect users from all bad links - that should be far more important than "PR sculpting". I feel that the nofollow tag is bad in that respect (it's complicated, maybe I'll do another post on that in my blog sometime).

What has worked best for me is finding sites that are ranking well and entering their url in the yahoo site explorer. It will give you a count on all of their dofollow links and you can just isolate the blogs from that list.

The penalizing part is really outdated myth. Google never penalizes for bad backlinks or comment post links. If that was a case people would spam around their competitor web sites. However, you will never receive any substantial benefit form it, besides the lost time.

Sometimes you can also use dofollow search engines to simplify your task. For instances.http://w3ec.com/dofollow/
Just use seo quake to confirm whether the links do not have the rel=nofollow tag added.

when I search for blogs, I don't really look at whether it has a "nofollow" tag. the benefits of commenting on a blog is not just in acquiring backlinks but in creating a community with in bloggers as well...

Important point. I stopped finding these kind of blog.
no good blogs are dofollow. better visit good blog and comment there. You will learn something also.